Yea those were definitely not boos. Those were Heaaaaathhs (stolen from 'Heeeeeaappp' but w/e we have some kind of towel I'm sure).

That stadium was so dead quiet it was funny. When the announcers have to speak up to get over the crowd noise for the broadcast (Steelers, Ravens, Seahawks are the three that pop into my head immediately)--THAT is a loud (and in the case of the Seahawks, very loyal) fanbase.